Field of the Invention
This invention relates to bioresorbable implants and methods of using such implants for treatments of neurologic disorders.
Description of the State of the Art
This invention relates generally to treatment of neurologic disorders with endoprostheses that are adapted to be implanted in the central nervous system. An “endoprosthesis” corresponds to an artificial device that is placed inside the body.
Neurologic disorders include Huntington disease, Parkinsons's disease, Alzheimer's disease, and brain neoplasms such as tumors, gliomas and meningiomas. Such neurological diseases are typically not treated with endoprostheses, i.e., implantation of an endoprosthesis in the neurovasculature. Therapeutic treatment of many neurologic disorders has been primarily through systemic administration of active agents or surgery.
Patients with coronary artery disease are conventionally treated with percutaneous interventional procedures (angioplasty and stenting), coronary artery bypass grafting (surgery) and medications to improve blood flow to the heart muscle. In particular, stents are generally cylindrically shaped devices that function to hold open and sometimes expand a segment of a blood vessel or other anatomical lumen such as urinary tracts and bile ducts. A “lumen” refers to a cavity of a tubular organ such as a blood vessel. Stents are often used in the treatment of atherosclerotic stenosis in blood vessels, where “stenosis” refers to a narrowing or constriction of a bodily passage or orifice. In such treatments, stents reinforce body vessels and prevent restenosis following angioplasty in the vascular system. “Restenosis” refers to the reoccurrence of stenosis in a blood vessel or heart valve after it has been treated (as by balloon angioplasty, stenting, or valvuloplasty) with apparent success.
Stents are typically composed of a scaffold or scaffolding that includes a pattern or network of interconnecting structural elements or struts, formed from wires, tubes, or sheets of material rolled into a cylindrical shape. This scaffold gets its name because it physically holds open and, if desired, expands the wall of a passageway in a patient. Typically, stents are capable of being compressed or crimped onto a catheter to a reduced diameter so that they can be delivered to and deployed at a treatment site.
Delivery includes inserting the stent through small lumens using a catheter and advancing it to the treatment site. Deployment includes expanding the stent to a larger diameter once it is at the desired location. Mechanical intervention with stents has reduced the rate of restenosis as compared to balloon angioplasty.
Stents are also used as vehicles for providing biological therapy or drug delivery. Biological therapy uses medicated stents to locally administer a therapeutic substance. Effective concentrations at the treated site require systemic drug administration which often produces adverse or even toxic side effects. Local delivery is a preferred treatment method because it administers smaller doses of medication than systemic methods, but concentrates the drug at a specific site.
A medicated endoprosthesis may be fabricated by coating the surface of either a metallic stent or a polymeric scaffold with a polymeric carrier that includes an active or bioactive agent or drug. Polymeric scaffolding itself may also serve as a carrier of an active agent or drug.
In coronary applications in which the stent maintains patency of a vessel the stent must be capable of withstanding the structural loads, namely radial compressive forces, imposed on the stent as it supports the walls of a vessel. Therefore, a stent must possess adequate radial strength. Radial strength, which is the ability of a stent to resist radial compressive forces, relates to a stent's radial yield strength and radial stiffness around a circumferential direction of the stent. A stent's “radial yield strength” or “radial strength” (for purposes of this application) may be understood as the compressive loading, which if exceeded, creates a yield stress condition resulting in the stent diameter not returning to its unloaded diameter, i.e., there is irrecoverable deformation of the stent. When the radial yield strength is exceeded the stent is expected to yield more severely and only a minimal force is required to cause major deformation. Radial strength is measured either by applying a compressive load to a stent between flat plates or by applying an inwardly-directed radial load to the stent.
Some treatments with stents require its presence for only a limited period of time. Once treatment is complete, which may include structural tissue support and/or drug delivery, it may be desirable for the stent to be removed or disappear from the treatment location. One way of having a stent disappear may be by fabricating a stent in whole or in part from a material that erodes, resorbs or disintegrates through exposure to conditions within the body. Stents fabricated from biodegradable, bioabsorbable, bioresorbable, and/or bioerodible materials such as bioabsorbable polymers can be designed to completely resorb only after the clinical need for them has ended.